This invention is concerned with apparatus for and a method of accurately proportioning and mixing fluids and is particularly, but by no means exclusively, concerned with such apparatus and method used in dialysis systems, first for the accurate proportioning and mixing of the two fluids which constitute the component parts of dialysate solutions, and secondly, for controlling the proportionate relationship between the dialysate solution and the waste material drawn from the blood of a patient across the membrane of a dialysis cell in the classical hemodialysis procedure. It is of course to be appreciated in these two cited instances that while mixing of the fluids will occur, the primary purpose is not such mixing but rather the controlled proportioning of merging fluid streams.
In the past, mixing of fluids in predetermined ratios has often been effected by the utilization of two or more positive displacement pumps, each handling a separate fluid and delivering to a chamber in which mixing occurs. Typical of prior art arrangements are the structures shown in Arp's U.S. Pat. Nos. 3,530,872 and 3,530,873, both issued Dec. 29, 1970. These patents are primarily designed for utilization in respirator systems where oxygen is mixed with air and, as such, extremely close adherence to the desired proportions of fluids in the mixture is not as critical. However, from a consideration of those patents, it will be apparent that they cannot deliver a closely controlled mixture of fluids.
Referring, for example, to U.S. Pat. No. 3,530,872 which shows a double acting piston/cylinder unit for handling the oxygen of the mixture and of which the piston is ganged to a single acting piston/cylinder for handling the air or other fluid, it will be apparent, considering FIG. 2 of that patent, that the quantity of oxygen moved from one side to the other of the double acting piston as that piston moves from right to left, will be in excess of that which can be accommodated on the other side of the piston and therefore that excess oxygen must pass to the outlet and, of course, will not be mixed with the second fluid.
U.S. Pat. No. 3,530,873 shows a pair of double acting piston/cylinder units one of which meters the oxygen supply and the other of which meters air or another fluid, the pistons of the two units being ganged by a common piston rod.
Referring to FIG. 3 of the drawings of that patent, again it is apparent that the volume of oxygen delivered to the outlet line is greater in left to right movement of the pistons than it is in right to left movement, whereas exactly the opposite is true of the second fluid. As such, there is substantial error introduced in the proportions of the fluids in the mixtures produced by that system.
Malburg's U.S. Pat. No. 2,203,832 issued June 11, 1940 purports to show a system for accurately proportioning and mixing two liquids one of which is water and the other of which is embalming fluid. Since Malburg is concerned with a treatment of cadavers the strictest accuracy is probably not necessary but a consideration of this patent will reveal that error must occur in his proportioning device. Referring to FIG. 1 of the patent, it is clear that the amount of embalming fluid delivered to be mixed with water during right to left movement of the pistons will exceed that amount delivered to be so mixed on left to right movement of the pistons. Since there is a by-pass between opposite sides of the piston in the cylinder from which the embalming fluid is delivered and since an additional mixing piston/cylinder device is provided this error may be lessened to acceptable limits considering that the resultant mixture is utilized only on cadavers. Nonetheless, the error is present and no attempt is made to correct it.
Apparatus utilized in hemodialysis procedures for mixing liquids in accurate proportions is shown in Willock's U.S. Pat. No. 3,598,727 issued Aug. 10, 1971. In that apparatus a double acting piston/cylinder unit has a pair of piston rods which extend from opposite ends of the cylinder in which the first liquid (usually water) is handled and each one extends into a corresponding one of two cylinders connected to supply of a second fluid (a dialysate concentrate). The liquids are delivered through a spool valve to be mixed before being admitted to a dialysis cell.
While Willock's apparatus may be accurate, it of course requires multiple piston and cylinder units and complex valving to unite the liquids.